For many MR applications at higher field strengths, the local SAR is a limiting factor. The SAR deposition increases with higher field strength and limits the RF power, duty cycle, and flip angles usable, leading to a lengthening of scan acquisition time to meet designated SAR limits. For a single transmitter system, the SAR was relatively easy to calculate, as all antenna elements transmitted with the same amplitude, and a fixed phase shift between them. Furthermore, the RF pulse shapes required for the experiments are known and are stored in a shape library, associated with their SAR. With the advent of multi-transmit systems where each coil element has the potential to independently transmit its own amplitude and phase, SAR must be calculated on a per channel basis also considering the parallel RF transmission pulses, which can only be calculated based on additional information, for example, B1 maps, and are thus experiment/patient specific.
RF safety is a prerequisite for in vivo parallel transmission MRI scans, that is, scanning within SAR limits using multi-channel RF transmit coils must be guaranteed. Scans cannot be started unless they are “SAR safe.” In an MR system with multiple transmit channels, SAR reduced RF pulses can be calculated by incorporating electrical field information into an RF pulse design. In the past, methods have been used that construct RF pulses in consideration of known SAR hotspots that are common to every individual (e.g., the eyes). This is generally not sufficient for whole body imaging, as SAR hotspots can vary in both position and magnitude from patient to patient, and from RF pulse to RF pulse. Thus, an RF pulse sequence that limits SAR to acceptable levels in one patient may not be so limiting with respect to another patient. Moreover, RF sequences that accommodate a known static hotspot may inadvertently exacerbate unknown, patient specific hotspots at other locations.
One possible solution is to develop a worst-case scenario estimation of SAR that would be safe for all patients. This solution, however, would limit the allowed RF duty cycle so much that the MRI system would become seriously compromised for use in conjunction with in vivo parallel transmission scans. The ability to tailor a SAR calculation to the patient would be more beneficial than using a blanket scenario or known term for all patients.
One reason in particular that RF sequences are currently not constructed on a patient-by-patient basis, is that for clinically relevant spatially RF pulses (e.g. local excitation or zoom imaging), parallel transmission systems that can accelerate these kinds of RF pulses (TxSENSE) are required. For an RF sequence, an underlying prerequisite is the availability to efficiently estimate the SAR. Moreover, availability of patient related E-fields and patient position are highly desirable for an accurate SAR estimation (including global and local SAR values and optionally a SAR map.) Field data obtained by simulations differ to some degree from the actual fields in the scanner. Use of bio-mesh models for E-field simulations instead of the actual patient leads to a systematic error that is difficult to characterize. For a standard, single channel birdcage coil RF transmit assembly, the RF waveforms are identical for every Tx coil element and only a phase increment (e.g. 45° for 8 elements) exists. For multiple Tx coil elements, the calculation is more complex, as each channel may have a different but static amplitude and phase. In more complex scans, such as for 2D/3D spatially selective pulses, each channel may have dynamically changing amplitudes and phases.
In order to calculate all SAR types as specified in the standard (local and global) and optionally a SAR map of a patient for a multi-channel RF transmit system, (e.g., eight transmit channels) the system performs a high number of calculations (e.g. TeraFLOPs): as high as 1010 calculations or more, depending on the resolution of the model and cells used for the calculation. This process would take several minutes and cannot practically be carried out in real time as the patient waits inside the scanner for the actual diagnostic scan to begin.
The present application provides a new and improved magnetic resonance system, which overcomes the above-referenced problems and others.